The present invention relates to a micromechanical component and an appropriate manufacturing method. The present invention as well as the underlying problem are explained with respect to a micromechanical pressure sensor in the manufacturing technology of surface micromechanics, although they could theoretically be applied to any micromechanical structural components.
In known methods heretofore, there is a capacitative pressure sensor in surface micromechanical technologies (OMM) which uses a costly and tedious etching process for etching out sacrificial oxide, in order to prevent sticking of the diaphragm to the underlying cavity bottom (see also, T. Scheiter et al., Sensors and Actuators A 67 (1998), 211 -214). This etching process consists in a repetitive sequence of 10 sec etching intervals in HF gas and subsequent rinsing in nitrogen.
Piezoresistive pressure sensor elements in OMM technology with structured polycrystalline resistors have not been published up to now. In the known pressure sensor elements, the piezoresistive resistors are diffused into a monocrystalline silicon layer.
The known pressure sensors up to now are adapted to various pressure regions by varying the diaphragm size, since the thickness of the diaphragm is preselected by the particular process used.
The micromechanical component according to the present invention or the corresponding manufacturing method according to the present invention has the advantage compared to known attempts of a solution, that a simple design of a pressure sensitive micromechanical component having a membrane is created. Adaptation to different pressure regions can occur by changing a single process step, namely of the epitaxy thickness of the functional layer. Adaptation of the lithography masks, as with known methods, is not required.
One idea on which the present invention is based, is that, between the substrate and the functional layer a cavity is provided, which defines a diaphragm region of the functional layer, and below the diaphragm region on the substrate, one or a plurality of spacers are provided, to prevent adhesion of the diaphragm region to the substrate during deformation.
By using such expediently pyramid-shaped spacers in the cavity, the sticking problem during gas phase etching can be prevented. The spacers in the cavity even permit doing without costly gas phase etching processes for dissolving out the sacrificial layer from the cavity. During the etching process, the spacers prevent the diaphragm from being drawn to the bottom of the cavity by the surface tension of the water produced during etching, and sticking to it. Through this, the etching rate can be markedly increased, and thus the processing time reduced. This permits also arriving at the large lateral etching depths for this design in acceptable time.
According to a preferred further refinement, in the diaphragm region on the functional layer, and insulated by an insulating layer, polycrystalline, piezoresistive printed circuit traces are provided, made of semiconductor material.
According to another preferred improvement thereto, in the diaphragm region and/or the periphery of the diaphragm region, stoppered etching channels are provided for etching a sacrificial layer defining the cavity, the insulating layer in the region of the etching channel having corresponding holes whose sidewalls are covered by the material of the printed circuit trace. The insulating layer under the polycrystalline resistors is covered laterally by polycrystalline silicon in the region of the etching channels. Without this covering, the insulating layer under the polycrystalline resistors would be etched away too, during the etching away of the sacrificial layer of the cavity, whereby the resistors would lift off.
According to yet another preferred further refinement, the semiconductor material is silicon.
According to still another preferred further refinement, the sacrificial layer and the insulating layer are a first and second silicon dioxide layer.
A further underlying idea of the present invention is that the following steps are carried out for manufacturing a micromechanical component having a diaphragm, as for instance a pressure sensor: Preparation of a substrate from a semiconductor material; providing a sacrificial layer on the substrate; structuring the sacrificial layer so as to define a later-formed cavity having an overlying diaphragm region; epitactic provision of a functional layer made of the semiconductor material on the substrate having the structured sacrificial layer; providing an insulating layer on the functional layer; providing etching channels in the diaphragm region and/or in the periphery of the diaphragm region for etching the sacrificial layer; etching the sacrificial layer; sealing the etching channels; and providing one or more spacers to prevent sticking of the diaphragm region to the substrate caused by deformation below the diaphragm region onto the substrate.
During etching away the sacrificial layer of the cavity, the design according to the present invention requires great lateral etching depth. In order to reach acceptable processing time, a high etching rate is desirable. Subject to the process, this produces relatively much water. Without special measures being taken, this would cause the diaphragm to be drawn to the cavity bottom by surface tension. Because of the close touching of the two surfaces over a large area, strong cohesion forces would be created, which would prevent releasing of the diaphragm from the cavity bottom after evaporation of the water. The spacers proposed within the framework of the present invention prevent sticking of the diaphragm to the bottom of the cavity. The surface tension of the water can draw the diaphragm down only up to the point where it rests on the spacers. The area over which diaphragm and spacers touch is very small. The small cohesion forces resulting from this can be overcome by the inner tension of the diaphragm, i.e., the diaphragm snaps back after evaporation of the water.
The method delineated here makes possible relatively simple and cost-effective manufacturing, using existing OMM process steps. Using this design, a clear reduction in size of the sensor element is possible. A considerable advantage comes about because the sensor element is adapted to other pressure ranges merely by changing layer thickness. The epitaxy thickness essentially determines the thickness of the diaphragm, and thus, how much the diaphragm is bent by an applied pressure. A thicker diaphragm requires a higher pressure for attaining a certain amount of deformation, and thereby a certain output signal. Particularly, the sensor element is also suitable for higher pressures.
One design element represents the structuring forward of the sacrificial material. It creates an etching stop during etching out of the cavity sacrificial material. The lateral dimension of the cavity is defined by the sacrificial layer. That stops the etching process laterally, whereby the position of the diaphragm edges is exactly defined. The forward structuring of the sacrificial material permits, in addition, the definition of lateral etching channels outside the cavity. The channels speed up the etching out of the sacrificial material, because, in addition to the etching channels in the middle of the diaphragm, the sacrificial material is also etched out by the lateral channels.
According to a preferred further refinement, provision is made of polycrystalline, piezoresistive printed circuit traces made of the semiconductor material in the diaphragm region on the insulating layer.
According to a preferred further refinement, the etching channels are provided using the following steps: Forming of holes in the insulating layer; providing a layer made of the printed circuit trace material on the insulating layer having the holes; depositing a protective layer on the layer made of the printed circuit trace material; forming of holes in the protective layer within the holes; and transferring the holes into the functional layer to form the etching channels.
In keeping with another preferred further refinement, the protective layer, the insulating layer and the sacrificial layer will be made of the same material.
According to yet another preferred further refinement, the semiconductor material is silicon. Before providing the sacrificial layer, the following steps are executed: Providing a silicon nitride layer on the substrate; structuring the silicon nitride layer in such a way that spots of the silicon nitride layer remain in the cavity to be formed later; thermally oxidizing the substrate with the spots of the silicon nitride layer, so that, under the spots of the silicon nitride layer, spacers for preventing adhesion of the diaphragm region to the substrate, during deformation, are formed from non-oxidized substrate material; and removing the silicon nitride layer. Optionally, then, thermal oxidizing can still be performed, in order to increase the clearance between the tip of the pyramids and the upper edge of the oxide.
In accordance with still another preferred further development, after etching the sacrificial layer above the layer made of the printed circuit trace material, a sealing layer for sealing the etching channels is deposited, and structured in such a way that the etching channels are sealed by plugs made of the sealing layer.
According to another preferred further refinement, the printed circuit traces are structured from the layer made of the printed circuit trace material, after the sealing of the etching channels.